DRX in LTE Systems
Industry standards for the Long-Term Evolution (LTE) wireless communication system, as developed by the 3rd-Generation Partnership Project (3GPP), include specifications for the implementation of a Discontinuous Receive (DRX) mode of operation for user terminals (“user equipment,” or “UEs,” in 3GPP terminology) that have established a connection with the radio network and are thus in a so-called RRC_CONNECTED state. More particularly, the requirements for DRX operation are described in section 5.7 of the Medium Access Control (MAC) layer specifications in the 3GPP document “Evolved Universal Terrestrial Radio Access (E-UTRA); Medium Access Control (MAC) protocol specification,” 3GPP TS 36.321, v. 10.5 (March 2012).
The purpose of DRX is to allow the UE some sleep time, i.e., time during which it does not have to monitor the Physical Downlink Control Channel (PDCCH) for new transmissions. The time during which the UE must monitor the PDCCH is called Active Time—when UE is not in an Active Time it does not have to monitor the PDCCH. The Radio Resource Control (RRC) protocol defined by 3GPP documents is used to activate the DRX mechanism for a given UE and defines the periods when the UE is in Active Time.
The Active Time includes time when at least one of the following conditions is fulfilled:                When the On Duration Timer is running. In the beginning of each DRX cycle, the On Duration Timer defines how long the UE should monitor PDCCH and be active. There are two types of DRX cycles, long DRX cycles and short DRX cycles. Short cycles are followed only when there has recently been uplink or downlink activity, while long cycles are used otherwise. In the discussion that follows, the term “OnDuration time” refers to the time during which the On Duration timer is running.        When the Inactivity Timer is running. When the PDCCH indicates a new transmission in either the downlink or uplink, i.e., a downlink assignment or uplink grant, the Inactivity Timer is started, or re-started if it is already running.        When a Scheduling Request is pending. After sending a Scheduling Request, which is a request for uplink resources, the UE expects the base station (an “eNodeB” or “eNB,” in 3GPP terminology) to schedule it and to send an uplink grant on PDCCH.        When the Retransmission Timer is running. LTE uses hybrid automatic-repeat-request (HARQ) in both the downlink and uplink. In the LTE downlink, retransmissions are asynchronous and are thus not always completed one HARQ round trip time (RTT) after the previous transmission. (This is different than the HARQ behavior for the uplink.) Thus, when the UE receives a downlink transmission, it starts a DL HARQ RTT Timer for the current HARQ process. When this timer expires, the Retransmission Timer of the HARQ process is started and the UE monitors the PDCCH for incoming assignments. The Retransmission Timer is started only when the UE has not been able to decode downlink data targeted to the UE, and has thus sent a negative acknowledgement (NACK) in the uplink.        When an uplink grant for a retransmission may occur. In LTE, the eNB may send a new uplink resource allocation together with the synchronous HARQ feedback, with the new uplink resource allocation specifying the uplink resources to be used for the retransmission. Note that during this subframe, which occurs 4 milliseconds after the initial uplink transmission, the UE should monitor not only the PDCCH, for uplink grants, but also the Physical Hybrid Indicator Channel (PHICH), for the HARQ feedback.        When an uplink grant is expected after receiving a Random Access Response, or when the Contention Resolution Timer is running.        
Note that regardless of Active Time, the UE should transmit or receive HARQ feedback when either is expected. Note also that in LTE networks that employ Time-Division Duplexing (TDD), the PDCCH subframe can be a normal downlink subframe or a special subframe, the latter of which includes a downlink part referred to as DwPTS. An uplink-only subframe in a TDD deployment cannot be a PDCCH subframe.
DRX in Systems Using Carrier Aggregation
In systems using a technique called carrier aggregation, one or more component carriers are aggregated together for a single UE, to provide wider bandwidth up to 100 MHz and higher bit rates, up to 3 Gigabits per second. In an LTE system employing carrier aggregation, a UE has one primary serving cell (PCell) and one or more secondary serving cells (SCells). Using RRC, the network configures, for each UE, which cell is acting as the UE's PCell and which cell or cells are acting as the UE's SCells. A configured SCell can be dynamically activated or deactivated, as needed. This allows additional bandwidth to be made available to the UE when needed, while allowing power consumption by the UE to be reduced when the additional carriers are not needed. The activation state is controlled by MAC Control Elements (MAC CEs) and timers. In Release 10 of the 3GPP standards for LTE, cross-carrier scheduling was also introduced, meaning that one cell may carry scheduling information on PDCCH for another cell. Which serving cell is scheduling a serving cell is configured semi-statically, using RRC.
In LTE Release 10, there is only one DRX mechanism, which includes one set of DRX timers common to all carriers/cells. This means that, when in DRX Active Time, the UE shall monitor the PDCCH of all activated serving cells. Furthermore, if, for example, the DRX-InactivityTimer is started in response to activity on one cell, the UE needs to monitor PDCCH on all serving cells.
CSI Reporting in LTE
In specifications for LTE, the term Channel State Information (CSI) is used as shorthand for control information that may include a Channel Quality Indicator (CQI), a Precoding Matrix Indicator (PMI), a Rank Indicator (RI) and a Precoding Type Indicator (PTI). CSI is used by the UE to provide channel quality estimations for the eNB.
CSI can be sent both as aperiodic and periodic control signals. Aperiodic CSI reports are always in response to an explicit request from the eNB, and are sent together with user data on the Physical Uplink Shared Channel (PUSCH). Periodic CSI reports are configured by RRC, to be sent periodically during the Active Time of the UE. Periodic CSI reports can be sent either on PUSCH, together with user data, or sent on the Physical Uplink Control Channel (PUCCH).
SRS Transmission in LTE
The sounding reference signal (SRS) is a reference signal sent by the UE in the uplink, and is used by the eNB to estimate the uplink channel quality. SRS can be sent both aperiodically or periodic. Aperiodic transmission of SRS is always in response to a request from the eNB. Periodic transmission of SRS is according to RRC configuration of the UE, and is sent periodically during the Active Time of the UE.
The use of carrier aggregation in systems that employ DRX causes additional complications for DRX, and may reduce the effectiveness of DRX. One reason for this is that a single DRX mechanism applies to all activated carriers. Improved techniques for managing transmissions during DRX mode are needed.